1. Field of the Invention
The present invention relates to a wireless communication apparatus suitable for a cell phone or wireless LAN apparatus.
2. Description of the Related Art
In a CSMA/CA type wireless communication system represented by wireless LAN communication defined by IEEE 802.11 in conventional wireless communication systems, if an acknowledgement frame (Ack frame) for transmission data cannot be received, the transmission data is retransmitted. In this case, on the basis of the retry count and lifetime unique to each transmission data, retransmission is limited by using the retry count and transmittable time of the transmission data.
In the QoS (Quality of Service)-extended wireless LAN standard IEEE 802.11e, communication is separately performed in two periods, i.e., a contention-based period during which each terminal station performs distributed access by using a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method, and a contention-free period during which a base station performs centralized control. The latter access control method using centralized control is called an HCCA (HCF Controlled Channel Access) method, and the former access control method using distributed control is called an EDCA (Enhanced Distributed Channel Access) method. A wireless LAN terminal is able to access a medium by using one of these access control methods, acquire TXOP (transmission opportunity) representing a period during which a plurality of data can be transmitted, and communicate data during the TXOP period (see IEEE 802.11e Draft 13.0, IEEE P802.11e/D13.0, January 2005 below).
In IEEE 802.11n aiming at high-speed transmission has proposed a method in which a terminal having acquired a TXOP period gives a part of this TXOP period to the data receiving terminal, and bi-directional communication is performed by a piggy back method in the TXOP period, thereby increasing the transmission efficiency. This method is called a bi-directional data flow or reverse direction.
In IEEE 802.11n, unlike in the existing IEEE 802.11 standard, an aggregation frame is formed by collecting (i.e., aggregating) a plurality of data into one data frame, and transmitted as one data frame, thereby reducing the overhead existing between individual data frames (when they are not aggregated).
To acquire a TXOP period for transmitting an aggregation frame in the EDCA method, the transmitting terminal (initiator) transmits an IAC frame, and the receiving terminal (responder) returns an RAC frame when SIFS has elapsed after that, thereby performing IAC-RAC frame exchange. Alternatively, RTS-CTS frame exchange defined by IEEE 802.11 may also be used instead of IAC-RAC frame exchange.
When IAC-RAC frame exchange is to be performed in the bi-directional data flow, the data receiving terminal notifies a data frame length and transmission data rate, which the terminal can transmit when given a part of a TXOP period, by writing the data in an RAC frame.
On the basis of the values written in the RAC frame, the transmitting terminal determines that part (RDG duration: reverse direction communication permission period) of the TXOP period, which is to be given after an aggregation frame is transmitted. The transmitting terminal writes the determined RDG duration in an IAC frame, attaches the IAC frame to the head of the aggregation frame, and transmits the aggregation frame when SIFS has elapsed after the RAC frame is received. The receiving terminal having received the aggregation frame having the IAC frame attached to the head must notify the reception status by a Block Ack (block acknowledgement) frame when SIFS has elapsed after the aggregation frame is received from the transmitting terminal. When the bi-directional data flow is used, the receiving terminal transmits the Block Ack frame when SIFS has elapsed by using the piggy back method by which several data frames are transmitted as they are aggregated in the Block Ack frame, thereby simultaneously transmitting the data and Block Ack frame. In this case, the transmission time of the aggregation frame formed by aggregating several data frames in the Block Ack frame cannot exceed the time of the RDG duration written in the IAC frame.
In this manner, a part of the TXOP period acquired by the transmitting terminal can be given to the receiving terminal.
If the receiving terminal further requests an RDG duration when transmitting an aggregation frame by the piggy back method, the receiving terminal can further request an RDG duration by writing, in an RAC frame, a data frame length and transmission data rate prepared for transmission, and returning the RAC frame by attaching it to the head of the aggregation frame to be transmitted by the piggy back method (see TGn Sync Proposal Technical Specification, IEEE 802.11-04/889r4, March 2005).
Also, JP-A 2003-60562 (KOKAI) below describes that retransmission is controlled by adjusting the signal length of a burst signal in burst communication by radio.
The following problem arises if the retransmission limiting method for each transmission data defined in the existing IEEE 802.11 is applied to the retransmission limiting method for burst transmission such as the Block Ack method defined in the conventional IEEE 802.11e. That is, if data in which error has occurred by burst transmission is transmitted simultaneously with new transmission data, burst data transmissions are excessively concentrated to the same terminal.
Also, when the transmission opportunities of data having a plurality of priority degrees are grouped in accordance with the priority degrees as defined in the conventional IEEE 802.11e, the transmission opportunities are excessively given to the same priority degree.
The above problem similarly arises in the aggregation method by which a plurality of transmission data are aggregated into one frame as burst data. The problem also similarly arises in the bi-directional data flow method by piggy back in which a part of the acquired TXOP period is given to the receiving terminal.
Furthermore, a new problem of the bi-directional data flow method is that although data are transmitted from both the transmitting side and receiving side in the bi-directional data flow method, if no transmission error occurs in the data transmitted from the transmitting side and an error occurs in only the data transmitted from the receiving side, the transmitting side need not retransmit the data. Therefore, no retransmission band is allocated to the receiving side, so the data in which the error has occurred is not retransmitted unless the receiving terminal reacquires the transmission right.
In addition, when data transmission is performed from a terminal station to a base station in the HCCA method of IEEE 802.11e, a QoS Cf-poll frame is transmitted to give the terminal station the transmission right of a TXOP period, and the terminal having acquired the transmission right transmits data during this TXOP period. If a transmission error or the like occurs in the TXOP period and TXOP reallocation is immediately performed, TXOP allocations may be concentrated to the same terminal.